IFF Mark III, also known as ARI.5025 in the UK or SCR.595 in the US, was the Allied Forces standard identification friend or foe (IFF) system from 1943 until well after the end of World War II. It was widely used by aircraft, ships, and submarines, as well as in various adaptations for secondary purposes like search and rescue. 500 units were also supplied to the Soviet Union during the war.
Mark III replaced the earlier Mark II which had been in service since 1940. Mark II had an antenna that received signals from radar systems, amplified them, and returned them. This caused the blip on the radar display to become larger, indicating a friendly aircraft. As the number of radar systems on different frequencies proliferated through the mid-war period, the number of models of Mark II had to do the same. Aircraft could never be sure their IFF would respond to the radars they flew over.
Freddie Williams had suggested using a single separate frequency for IFF as early as 1940, but at that time the problem had not become acute. The introduction of microwave radars based on the cavity magnetron was the main impetus for adopting this solution, as the Mark II could not easily be adapted to respond on these frequencies. In 1942, a new frequency band, between 157 and 187 MHz, just below most VHF radars, was selected for this role. The only downside to this design is that the radar itself no longer provided the trigger signal for the transponder, so a separate transmitter and receiver was needed at the radar stations.
The Mark III began to replace the Mark II in 1942 and 1943, in a somewhat lengthy switchover period. It was also used as the basis for several other transponder systems such as Walter and Rebecca/Eureka, which allowed suitably equipped aircraft to home in on locations on the ground. These found use for dropping paratroopers and supplies in Europe, locating downed aircraft, and other roles. Several newer IFF designs were trialled, but none of them offered enough of an advantage to warrant a switchover. Mark III was replaced by IFF Mark X over an extended time starting in 1952.